A liquid crystal display device is widely used as a display device of various information processing apparatuses such as a computer and a television set. Particularly, the liquid crystal display device of a TFT system (also called “TFT-LCD” hereafter) is widely spread, and expansion of a further market thereof is expected, and with expansion of the market, further improvement of an image quality is desired. The TFT-LCD is taken as an example for explanation hereafter. However, the present invention is not limited to the TFT-LCD, but covers the whole field of the liquid crystal display device, and can be applied to the liquid crystal display device of a passive matrix system and a plasma address system for example.
A so-called TN (twisted nematic) mode is a display mode that has been most widely used in the TFT-LCD heretofore, in which a liquid crystal having positive dielectric anisotropy is horizontally aligned between mutually opposing substrates. A TN mode liquid crystal display device is characterized in that a direction of an alignment of liquid crystal molecules adjoining one of the substrates is twisted by 90° with respect to a direction of an alignment of liquid crystal molecules adjoining the other substrate. In such a TN mode liquid crystal display device, although an inexpensive manufacturing technique is established and industrially full growth is achieved, it is difficult to realize a high contrast ratio.
Meanwhile, a so-called VA mode liquid crystal display device is known, in which liquid crystals having negative dielectric anisotropy are vertically aligned between the mutually opposing substrates. In the VA mode liquid crystal display device, liquid crystal molecules are aligned approximately vertically to a substrate surface, when not applying a voltage. Therefore, almost no rotatory polarization and birefringence are exhibited by the liquid crystal cells, thus allowing lights to pass through the liquid crystal cells with almost no change in its polarization state. Accordingly, approximately perfect black display can be realized when not applying the voltage, by arranging a pair of polarizers (linear polarizers) on upper and lower sides of the liquid crystal cells so that absorption axes thereof are crossing each other orthogonally (also called “crossed Nicols polarizers” hereafter). When applying the voltage of a threshold value or more (abbreviated as “at a voltage application time” hereafter), the liquid crystal molecules are tilted so as to be approximately parallel to the substrate, thus exhibiting a great birefringence and realizing a white display. Accordingly, such a VA mode liquid crystal display device can easily realize a significantly high contrast ratio.
In such a VA mode liquid crystal display device, asymmetry (skewness) is generated in a viewing angle characteristic of the liquid crystal display device, if the liquid crystal molecules are tilted in one direction at the voltage application time. Therefore, an domain division type VA mode is widely used, in which an tilt direction of the liquid crystal molecules is divided into a plurality of directions in a pixel, for example by devising a structure of a pixel electrode, or by providing an alignment control part such as protrusions in the pixel. Note that each region with different tilt azimuths of the liquid crystal molecules, is called a domain, and the domain division type VA mode is also called a MVA mode (multi-domain type VA mode).
In the MVA mode, from a viewpoint of maximizing transmissivity in a white display state, usually an axial azimuth of the polarizer is set to be an angle of 45° with respect to the tilt azimuth of the liquid crystal molecules at the voltage application time. This is because the transmissivity at the time of interposing a birefringent medium between the crossed Nicols polarizers, is proportional to sin2 (2α) when an angle formed by an axis of the polarizer and a slow axis of the birefringent medium is defined as α (unit: rad). In a typical MVA mode, the tilt azimuth of the liquid crystal molecules can be divided into four domains of 45°, 135°, 225°, and 315°. In such a MVA mode in which the tilt azimuth is divided into four domains, Schliere alignment or an alignment in an unintended direction is frequently observed on a boundary between domains or near the alignment control part, thus causing transmissivity loss.
In order to solve the above-described problem, the VA mode liquid crystal display device using a circularly polarizing plate, is examined (for example, see Patent Document 1). According to such a liquid crystal display device, the transmissivity at the time of interposing the birefringent medium between right and left circularly polarizing plates crossing each other orthogonally, does not depend on an angle formed by the axis of the polarizer and the slow axis of the birefringent medium, and therefore desired transmissivity can be ensured if only controlling the tilt of the liquid crystal molecules even in a case that the tilt azimuth of the liquid crystal molecules is set to be an angle excluding 45°, 135°, 225°, and 315°. Accordingly, for example, a circular protrusion may be disposed in a center of the pixel, so that the liquid crystal molecules are tilted in all azimuths, or the liquid crystal molecules may be tilted in random azimuths without controlling the tilt azimuth at all. Note that the VA mode using the circularly polarizing plate is also called a circularly polarizing VA mode or a circularly polarizing mode. Meanwhile, the VA mode using a linearly polarizing plate is also called a linearly polarizing VA mode or a linearly polarizing mode. Further, as is publicly-known, the circularly polarizing plate is formed typically by combining the linearly polarizing plate and a λ/4 plate.
Further, the circularly polarized light has a performance that right and left chirality is replaced with each other when being reflected by mirror, etc. For example when a left circularly polarizing plate is disposed on the mirror and a light is incident thereon, the light transmitting through the circularly polarizing plate and converted to a left circularly polarized light, is reflected by the mirror and converted to a right circularly polarized light, and the right circularly polarized light can't transmit through the left circularly polarizing plate. Thus, it is known that the circularly polarizing plate has an antireflective optical function. It is also known that there is an effect of improving a bright room contrast ratio of the display device such as a VA mode liquid crystal display device, because the antireflective optical function of the circularly polarizing plate contributes to preventing unnecessary reflection when the display device is observed in a bright room environment like outdoors. Here, it can be considered that the unnecessary reflection is mainly caused by interior components of the display device such as a transparent electrode and metal wiring of a TFT element. If the unnecessary reflection is not prevented, light quantity at the time of a black display by the display device becomes large when observed in a bright room environment, even in a case of the display device realizing approximately perfect black display in a dark room environment, resulting in reducing the contrast ratio.
As described above, a transmissivity improvement effect and an unnecessary reflection prevention effect can be obtained in the circularly polarizing VA mode using the circularly polarizing plate. However, the liquid crystal display device of the circularly polarizing VA mode has a room for improvement in a point that the contrast ratio is reduced at an oblique viewing angle, and a sufficient viewing angle characteristic can't be obtained. Meanwhile, various improved techniques of the viewing angle characteristic using a birefringent layer (phase difference film) are proposed. For example, Patent Document 1 discloses a method described in the following (A), Patent Document 2 discloses a method described in the following (B), Patent Document 3 discloses a method described in the following (C), Patent Document 4 discloses a method described in the following (D), and Non-Patent Document 1 discloses a method described in the following (E).
(A) A method using two λ/4 plates satisfying a relation of nx>ny>nz.
(B) A method using two λ/4 plates satisfying a relation of nx>ny>nz, and one or two second-type birefringent layers satisfying a relation of nx<ny≦nz, by combination.
(C) A method using two λ/4 plates satisfying a relation of nx>nz>ny, and a third type birefringent layer satisfying nx=ny>nz, by combination.
(D) A method according to the method of (C) further using one or two λ/2 plates satisfying a relation of nx>nz>ny, by combination.
(E) A method using two uniaxial λ/4 plates (so-called A-plate satisfying a relation of nx>ny=nz), and a third type birefringent layer satisfying a relation of nx=ny>nz, and a birefringent layer satisfying a relation of nx>nz>ny, by combination.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-40428
Patent Document 2: Japanese Patent Application Laid-Open No. 2009-37049
Patent Document 3: Japanese Patent Application Laid-Open No. 2003-207782
Patent Document 4: Japanese Patent Application Laid-Open No. 2003-186017    Non-Patent Document 1: Zhibing Ge, and six others, “Wide-View Circular Polarizers for Mobile Liquid Crystal Displays”, IDRC08, 2008, p. 266-268
However, as a result of an examination by inventors of the present invention, it is found that the viewing angle characteristic still has a room for improvement even in the methods of (A), (B), and (C). Further, the viewing angle characteristic also has a room for improvement in the methods (C), (D), and (E), in a point that an expensive biaxial phase difference film is required, which is difficult to be manufactured, requiring a high cost, and satisfying a relation of nx>nz>ny (satisfying a relation of 0<Nz<1).